1. Field of the Invention
The present invention relates to a marine drilling riser provided with improved buoyancy cans. In one aspect, the invention relates to an improved buoyancy can having vent means for rapidly venting compressed gas.
2. Description of the Prior Art
A substantial amount of exploratory drilling for deposits of crude oil and natural gas situated offshore is conducted from floating vessels. Such operations normally employ a marine riser which extends between the vessel and the subsea well. The riser consists of a number of sections of pipe connected together in end-to-end relation and serves to guide the drill string into the well and conduct drilling returns back to the vessel.
The riser must be supported in tension to prevent buckling due to several forces including (a) its own weight, (b) pressure differential caused by heavy drilling fluid, and (c) forces acting on it as a result of waves, current, and the like. Such support is normally provided by tensioning devices mounted on the drilling vessel and external buoyancy means connected to the riser. External buoyancy devices include buoyancy cans and/or modules longitudinally spaced along the riser pipe.
Positively buoyant risers of this construction present a serious hazard. In the event the riser breaks, the riser will be propelled upwardly by the buoyancy cans and possibly torpedo the drilling vessel or other work vessels in the area. This hazard has severely limited the use of a positively buoyant riser. Instead, buoyancy cans are designed to provide only a portion of the buoyancy needed to support the riser, the remaining riser support being provided by cumbersome and complex tensioning devices positioned on the vessel.
Another problem associated with buoyant risers and self-standing risers is that of maintaining sufficient tension on the riser when it is disconnected from the subsea wellhead. Disconnection of the riser may subject the riser to high lateral stress due to subsea currents. To offset these forces, the riser should be in tension due to its own weight. However, the buoyancy imparted by the buoyancy cans may reduce the net weight of the riser sufficiently to cause the riser to be damaged by sea currents.
In summary, there exists a need to rapidly eliminate or reduce the buoyancy on risers when the riser fails or is disconnected from the wellhead.
U.S. Pat. No. 3,992,889 discloses a buoyancy can provided with means for bleeding gas from the buoyancy chamber. However, the purpose of the bleed means is to adjust buoyancy in the chamber and, because of its structure, the bleed means would not operate within a time frame required to avoid the hazards described above.